CVE-2026-113799
MediumCVSS 5.3Exploitation Probability (EPSS)
Low risk37th percentile - higher than 37% of all known CVEs
Summary
Slack Morphism client library versions 1.0.0 through 2.27.0 expose the correct Slack request HMAC when signature verification fails. For an invalid request, the library computes the correct signature for the attacker-controlled timestamp and body and stores it in SlackEventWrongSignatureError.generated_hash, which the default Axum error handler logs. An attacker who can reach the callback endpoint and read logs can use the verifier as an HMAC oracle and resend the request with a valid signature within the five-minute window.
Risk Assessment
An attacker with access to application logs can bypass Slack signature verification and reach the protected callback without knowing the signing secret. This enables unauthorized invocation of protected application logic.
Recommendation
Upgrade Slack Morphism to version 2.28.0 or later. Restrict access to application logs and consider disabling logging of the generated_hash value.
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Original NVD description (English source)
Slack Morphism is a client library for Slack Web/Events API/Socket Mode and Block Kit. Versions 1.0.0 through 2.27.0 expose the correct Slack request HMAC when signature verification fails. For an invalid request, `slack-morphism` computes the correct signature for the attacker-controlled timestamp and request body and stores it in `SlackEventWrongSignatureError.generated_hash`. The error's `Debug` and `Display` representations expose that value, and the default Axum listener error handler logs it at error level. An attacker who can both reach the application's Slack callback endpoint and read its application/service logs can therefore use the verifier as a chosen-message HMAC oracle. The attacker can send an arbitrary request with an invalid signature, recover the correct signature from the resulting log entry, and resend the same request within the five-minute freshness window. The second request passes Slack Morphism's signature verification and reaches the protected application callback without the attacker knowing the Slack signing secret. Version 2.28.0 patches the issue.
Vulnerability data from NVD (NIST) · CISA KEV · EPSS

